This invention relates to methods for making coated papers wherein the coating contains thermoplastic binders and more particularly to method wherein the coated paper is subjected to an efficient multiple stage drying sequence prior to calendering to produce a coated paper having high bulk, excellent gloss and low surface mottle.
The methods for producing a coated paper are well known. An aqueous coating composition is continuously applied to a moving web, the coating composition typically containing one or more pigments such as clay, and an organic binder. The coated paper is then dried and subjected to a calendering operation to produce the desired level of gloss. Conventional methods of calendering include supercalendering and gloss calendering.
In some instances, it is desirable to obtain a finished coating that has a high degree of bulk together with acceptable gloss. High bulk papers are usually finished by gloss calendering wherein the coated web is brought between the nip of a heated, polished roll and a resilient backing roll, using relatively low pressures and high temperatures compared to supercalendering. As the coated surface of the paper contacts the heated drum, the binder is softened and the coating is smoothed without undue compacing of coating.
In the art of applying high gloss coatings, it is known to utilize thermoplasic polymers having relatively high minimum film forming temperature or glass transition temperature, e.g., in the order of 95.degree. to 135.degree. F. The minimum film forming or glass transition temperature of a binder is generally the temperature above which the binder particles start to melt or become plastic. The use of such binders is generally described in the following U.S. Pat. Nos. 3,028,258; 3,268,354; 3,583,881; 3,664,912; 3,873,345; 4,012,543; 4,102,737 and 4,112,192.
The Hain U.S. Pat. No. 3,268,354 describes the preparation of coated papers wherein the coating may contain pigment and a thermoplastic binder. The coated paper is then partially dried at temperatures below the minimum filmforming temperature of the binder such that the binder particles in the coating remain uncoalesced. The paper is calendered while substantially wet, causing significant pressure deformation of the paper web, and also causing the binder to fuse and form a glossy film with the pigment.
Other references that teach the advantages of providing a coating wherein the binder is uncoalesced are U.S. Pat. Nos. 3,873,345 and 4,112,192. In order to maintain the binder in an uncoalesced state, the temperaure during drying is kept below the minimum film forming temperature of the binder. In accordance with some of the other aforementioned references, the binder is coalesced while drying at a relatively high temperature before calendering.
It has been found that drying at a temperature below the minimum film forming temperature of the binder is inefficient because of the excessive time required in the dryer. On the other hand, the use of conventional higher drying temperatures does not result in the development of the desired degree of gloss in the final product.